The present invention relates to a superconductive coil system for use in a nuclear fusion plant or a magnetic train floating system, and more particularly to a superconductive coil system which is equipped with a protecting device in a quenching operation.
As a protecting device when in the quenching operation of a superconductive coil system, there has been used a resistor type protecting device which is disclosed in Japanese Patent Laid-Open Publication No. 55-27410 laid open in 1980. In the system thus disclosed, the superconductive coil is energized by a d.c. motor through a current switch when in a normal running operation. In the quenching operation, on the contrary, the current switch is turned off so that a portion of the magnetic energy owned by the superconductive coil is consumed as the Joule heat of a protecting resistor which is connected in parallel with the superconductive coil. As a result, the heat generation in terms of the Joule heat of the superconductive coil is reduced.
Both the current to flow through the superconductive coil after the current switch is turned off and the terminal voltage e are expressed by the following Equation: EQU L(di/dt)+Ri=0
hence, EQU i=I.sub.0 e.sup.-(R/L)t ( 1)
hence, EQU e=E.sub.m.e.sup.-(R/L)t ( 2)
wherein:
E.sub.m =I.sub.o R.sub.c : Maximum coil terminal voltage; PA1 R=R.sub.c +R.sub.s ; PA1 L=L.sub.s ; PA1 L.sub.s : Inductance of the superconductive coil; PA1 R.sub.s : Resistance after the quenching operation of the superconductive coil; PA1 R.sub.c : Resistance of the protecting resistor; PA1 I.sub.0 : Energizing Current in the normal operation; PA1 t: Time.
The ratio P.sub.r of the energy to be consumed by the superconductive coil to the magnetic energy of the superconductive coil is expressed by the following Equation if the resistance R.sub.s of the superconductive coil after the quenching operation is assumed to be constant: ##EQU1##
Since the maximum coil terminal voltage E.sub.m is restricted by the insulating characteristics (or the withstand voltage performance) of the superconductive coil, the resistance of the protecting resistor has to be expressed by the relationship of R.sub.c .ltoreq.E.sub.m /I.sub.0.
On the other hand, the heat to be generated by the superconductive coil is reduced the more, in view of the Equation (3), as the resistance R.sub.c of the protecting resistor becomes the more than the resistance R.sub.s of the superconductive coil when in the quenching operation. Therefore, the resistance of the protecting resistor is preset as high as possible within the limit of the withstand voltage performance of the superconductive coil.
However, since the allowable withstand voltage of the superconductive coil is generally low, e.g., 1 to 2 KV at the highest, there arises a defect that the heat generation percentage of the superconductive coil becomes high and reaches 20 to 30% or more.
Moreover, since the Joule heat loss takes only at the superconductive coil for the time period from the instant when it is detected in terms of the detection of the terminal voltage or the density of magnetic flux that the superconductive coil is quenched to the instant when the current switch is turned off, i.e., for 0.1 to 0.3 seconds, there arises another defect that the self-generation of the heat is increased.